1. Field of the Invention
The present invention relates to a motor control technology for controlling as desired the number of revolutions or a torque of an alternating current motor, such as an induction motor and a permanent magnet synchronous motor, in an extremely low speed region close to zero speed, and more particularly, to a drive device for an alternating current motor and an electric motor vehicle using sensorless control, in which a rotational position angle and speed information of a rotor of the electric motor are not detected directly by a sensor.
2. Description of the Related Art
In order to control an alternating current motor in a highly responsive and accurate manner, it is necessary to manage a torque of the electric motor. For this purpose, it is necessary to detect the number of revolutions (in the case of a synchronous motor, position (angle) information of a rotor) of the electric motor by means of a sensor, and further detect a phase current of the electric motor. After that, torque current components and excitation current components contained in the current need to be separated and controlled independently of each other inside of a controller. This technology is widely used as vector control.
In the vector control, speed (or position) information of the rotor is essential, and on the other hand, there arise problems of a reduction in reliability, an increase in the amount of work for attachment and adjustment, an increase in costs, and the like due to sensors for the essential information. As a solution to the problems, a sensorless control technology which does not directly use the speed or position information of the rotor has been developed, and has already been put into practical use. According to the sensorless control technology, stable driving can be achieved by a method based on an induced voltage of the electric motor in a region in which the rotational speed is high, whereas it is extremely difficult to realize the vector control in a region close to zero speed in which the induced voltage becomes smaller. For example, conventional examples as disclosed in the following patent documents can be given as the sensorless control technology for the region close to zero speed.
The following patent documents each relate to a method of utilizing the saliency of the rotor of the alternating current motor, in which a property that a change in a rotor position changes an inductance is utilized. For this reason, the saliency of the electric motor is essential. Japanese Patent Laid-Open Publication No. 2001-286198 (Patent Document 2) is directed to an induction motor, magnetic saturation of an iron core is generated also in the induction motor by causing an excitation current to flow therethrough, and the saliency is slightly generated in a magnetic circuit, so that the generated saliency can be utilized.
According to methods disclosed in Japanese Patent Laid-Open Publication Nos. 07-24598, 2001-286198, and 2002-291283 (Patent Documents 1 to 3), in order to observe the saliency as described above, a harmonic voltage is intentionally applied to the electric motor to thereby give variations to a current, and a magnetic flux position inside of the electric motor is estimated on the basis of the current variation amount.
For example, in a representative drawing (FIG. 1) of Japanese Patent Laid-Open Publication No. 2001-286198 (Patent Document 2), a high-frequency wave generator denoted by part number 4 generates a harmonic voltage, and the generated voltage is added to an output voltage Vγ* of a current controller. In addition, a current is of the electric motor is detected by a current sensor 12, and high-frequency components are taken out via a band-pass filter (BPF) 9. The magnetic flux position inside of the electric motor is estimated on the basis of the taken-out high-frequency current components.
In the case of the vector control of the alternating current motor, the linearization of a torque is a principal purpose, and the magnetic flux position inside of the electric motor may be detected for this purpose. In a permanent magnet synchronous motor, normally, the magnetic flux position and the rotor position coincide with each other. Therefore, if a rotor position sensor is provided, the magnetic flux position can be detected, but if no sensor is provided, a certain estimated calculation is required. Similarly in the case of an induction motor which is an alternating current motor other than the synchronous motor, if the magnetic flux position can be estimated, the degree of “slip” can be determined, whereby the vector control is established.
Japanese Patent Laid-Open Publication No. 2007-129844 (Patent Document 4) relates to an invention which has an object to reduce the amount of the applied harmonic voltage in Japanese Patent Laid-Open Publication Nos. 07-24598, 2001-286198, and 2002-291283 (Patent Documents 1 to 3). An external detector for a current change amount is provided, and the magnetic flux position is estimated and calculated on the basis of the detected current change amount. In the configuration of the detector for the current change amount, a current change rate of the electric motor is first detected, and the detected current change rate is integrated during the application period of the harmonic wave, to be thereby converted into the current change amount.
Japanese Patent Laid-Open Publication No. 2002-78391 (Patent Document 5) has basically the same perspective as those in Japanese Patent Laid-Open Publication Nos. 07-24598, 2001-286198, and 2002-291283 (Patent Documents 1 to 3), and describes a method of directly detecting the current change amount similarly to Japanese Patent Laid-Open Publication No. 2007-129844 (Patent Document 4). Patent Document 5 describes an example in which an external inductance is provided as a sensor for the current change rate.
Japanese Patent Laid-Open Publication No. 2001-169560 (Patent Document 6) utilizes the saliency of the electric motor similarly to Japanese Patent Laid-Open Publication Nos. 07-24598, 2001-286198, 2002-291283, 2007-129844, and 2002-78391 (Patent Documents 1 to 5). According to the invention disclosed in Patent Document 6, a harmonic voltage is applied to a phase assumed as the magnetic flux position of the electric motor, a harmonic current generated at this time is treated as a vector quantity, and the direction of the vector is controlled so as not to be displaced from a harmonic wave application direction, whereby the magnetic flux position of the electric motor and the estimated position for control are made coincident with each other.
In Japanese Patent Laid-Open Publication No. 08-205578 (Patent Document 7), unlike the other patent documents, the application of a harmonic wave is not performed basically. Ripples in a harmonic current generated by a normal PWM operation are detected, and a simultaneous equation is solved from the current ripples and a constant of the electric motor, whereby the magnetic flux position is estimated and calculated.
All of the methods disclosed in Japanese Patent Laid-Open Publication Nos. 07-24598, 2001-286198, 2002-291283, 2007-129844, 2002-78391, and 2001-169560 (Patent Documents 1 to 6) require the intentional application of a harmonic voltage. Returning to the principle of the vector control of the alternating current motor, the application of a harmonic voltage is not required originally, and rather, a large number of disadvantages occur as the drive device for the electric motor.
In FIGS. 29A and 29B, a comparison is made between a waveform of a line-to-line voltage applied to the electric motor when the application of a harmonic wave is not performed (FIG. 29A) and a waveform of a line-to-line voltage applied thereto when the application of a harmonic wave is performed (FIG. 29B). In ideal pulse width modulation, with respect to a cycle of a fundamental wave, pulses appear only on the positive side in a half cycle, and appear only on the negative side in the other half cycle, so that an amount of generated harmonic wave is minimized. However, if a harmonic wave is applied intentionally, the waveform as shown in FIG. 29B appears, and the amount of generated harmonic wave increases at once. In addition, the current waveform of the motor changes as shown in FIGS. 30A and 30B depending on whether or not a harmonic wave is applied.
The application of a harmonic wave as described above causes various problems. For example, electromagnetic noise from the electric motor increases. In order to increase the estimation sensitivity for the magnetic flux of the electric motor, it is necessary to reduce the frequency of the harmonic voltage or increase the amplitude thereof, and both cases lead to the increase in noise within an audible range (several 100 Hz to several kHz). In addition, a large amount of harmonic components is generated in a drive current for the electric motor, so that loss (mainly, iron loss) caused by the harmonic wave considerably increases. The increase in loss leads directly to a problem of heat generation, and thus becomes an extremely important factor in terms of the design of a motor drive system.
In Japanese Patent Laid-Open Publication No. 2007-129844 (Patent Document 4), a reduction in the harmonic voltage is realized, but the loss caused by the harmonic wave largely occurs even by a slight amount of application. In a normal pulse width modulation system (PWM system), carrier wave (carrier) frequency components, which are zero-phase components, are not generated, and only sideband wave components (neighboring the carrier wave) having a small amplitude are contained in the electric motor current. However, if the harmonic wave is applied intentionally, the applied components themselves directly become a harmonic current to be largely generated, so that a loss larger than expected occurs.
In Japanese Patent Laid-Open Publication No. 2002-78391 (Patent Document 5), although an example of detecting the current change rate is described, the contents of the invention are strictly an algorithm for “the current change amount”, and a specific method of directly utilizing the current change rate is not described.
In Japanese Patent Laid-Open Publication No. 08-205578 (Patent Document 7), although there is a description that the magnetic flux position is estimated from the current ripples without performing special PWM, in actuality, the special PWM for increasing a pulse width is required in a low speed region in which an output pulse width of an inverter becomes smaller, with the result that the harmonic current is increased. In addition, an external circuit for detecting the current ripples is essential, and this is not practical in consideration of ringing caused by switching of the inverter and the like.
Further, the simultaneous equation is solved from the constant of the electric motor inside of the controller, whereby the position of a magnetic pole is calculated. However, the nonlinearity of a magnetic circuit is strong in the electric motor of recent years, and hence it is becoming difficult to treat the constant of the electric motor as a “constant”. Therefore, it can be said that the direct use of this method is difficult.